This invention relates to an electromechanical actuator device for a sewing machine.
In particular, this invention relates to a device of the type comprising an input arranged to receive an electrical signal, and an output comprising a mechanical member, the movements of which depend on the said input signal and define the position of a needle bar and conveying foot in a sewing machine.
These actuator devices are classified as either analogue or logic, according to the type of electrical input signal.
Analogue actuators include the linear motor, in which the axial position of a respective slider is controlled by a closed loop electronic circuit. This circuit provides the motor with an electrical feed signal until the said slider reaches a predetermined position, as detected by means of a suitable sensor. The cost of such motors is generally relatively high, and this is increased by the cost of the relative closed loop control circuit.
Logic actuators include devices comprising a plurality of electromagnets which act on a mechanical decoder unit. Each electromagnet is fed with a logic signal having a predetermined "weight", and operates a corresponding element of the said mechanical unit, which produces a movement of a predetermined extent, proportional to the "weight" of said logic signal. As each electromagnet acts directly on a respective element of the mechanical unit, it is generally necessary to use electromagnets of high power, which consequently have a high current absorption, have operating times which are difficult to control, particularly in the case of high operating speeds, and are particularly nose.
Such electromechanical actuator devices are often used in sewing machines provided with electronic control units comprising microcomputers. These microcomputers produce logic output signals, and thus analogue actuator devices require the use of further digital-analogue converter circuits in such cases, with consequent further expense which is added to the already considerably high cost.